Optical components are extensively used in data communication networks. Typically these components are designed to manipulate light having a single mode. In order to increase the rate of information transfer associated with an individual optical waveguide it is known to multiplex optical signals with different, predetermined wavelengths. When many different wavelengths are multiplexed on a waveguide it is increasingly common, from a viewpoint driven by cost, space, power etc. to employ planar integrated circuit solutions to manipulate, combine or separate the individual optical signals rather than assemblies of discrete or hybrid natures. Even then, there are continued benefits from reducing the die footprint of the monolithic solution, such as reduced die costs, smaller packages, lower electrical power for thermal stabilization etc. It is commonly known to separate a wavelength multiplexed optical signal into a plurality of optical signals, each having a characteristic wavelength range. This application is ideally suited to integrated wavelength demultiplexers. Examples of such devices include echelle gratings and arrayed waveguide gratings, AWG for short. These devices are designed to support a single mode of light and typically, anything from a relatively small number of wavelength channels, typically four wavelength channels, through to complete high capacity long-haul systems with 80 wavelength channels. But, irrespective of the chosen material system, which can include for example silica glass, polymer, indium phosphide, gallium arsenide, and silicon, the guiding of the single mode of light is that of a weakly confined mode within the waveguide of such integrated optical waveguide devices.
Additionally, these devices incorporate a wavelength dispersive element that causes a wavelength multiplexed optical signal coupled to an input port of the device to separate into the individual optical signals corresponding to predetermined wavelength channels. Each of these individual optical signals is coupled to a specific output waveguide. Given the previous statements that minimizing the die dimensions is beneficial itself, even above and beyond simply migrating to a planar platform, the design of the wavelength dispersive element is therefore similarly driven to minimum dimensions as this is generally the dominant element of the overall monolithic circuit. Consequently, it is generally the case that the individual waveguides corresponding to these different waveguide channels are closely spaced and approximately parallel. Furthermore, subsequent elements operating in arrayed form on the discrete wavelengths benefit from similar closely spaced waveguides to minimize die footprint. Unfortunately, when optical waveguides, especially with weakly confined modes, remain closely spaced over a sufficiently long distance then the optical signals that propagate therein couple between adjacent waveguides and will therefore act essentially as a source of crosstalk. The amount of crosstalk being a function of the separation of the waveguides, the distance along which the waveguides are approximately parallel and the optical confinement of the waveguides for a given wavelength of the optical signal. Furthermore, the arrayed nature of the designs can result in said mode coupling in some degree from say a first channel to the second, but now this second channel is coupled to both first and third channels. This can result in optical crosstalk occurring across multiple waveguides from the originating source and works similarly across the entire array.
Additionally, when the wavelength multiplexed optical signal is dispersed in dependence upon wavelength, the optical signals provided often include optical modes other than the desired lowest order single mode in the receipt waveguides from the dispersive element. This is a result of adjusting the modal behaviour of the receipt waveguide so that the overall overlap of the structure generates a substantially flatter passband which is desirable in filtering elements separating optical channels. Since the waveguides are weakly confined, these higher order modes can continue propagating along the waveguides as the structure is tapered back towards a singlemode structure. Worse still, these higher order modes are positioned such that more of their energy is positioned further away from the waveguide center than the desired single mode and therefore such modes are more likely to couple to adjacent channels and cause crosstalk.
It is obvious to one trained in the art that if the waveguides on the substrate are made at a substantially non-zero angle to each other, making the waveguides sufficiently separated over relatively short distances, that cross talk between them becomes negligible. While this solution does reduce the undesired crosstalk it also results in significantly larger dimensions of the device, and in some material systems causes manufacturing issues with off-axis etching of materials. Increasing the chip size reduces the number of chips that are normally provided by a wafer thereby increasing costs of the finished device significantly.
Clearly, it would be beneficial to have a design for an integrated waveguide structure that incorporates an array of closely spaced parallel single mode waveguides, each of the waveguides for propagating an optical signal and each of the optical signals substantially confined to a region proximate to the waveguide core, thereby preventing crosstalk. Additionally, it would be beneficial if such a waveguide structure were produced using well-established, cost effective processes.